Introduction to Protein Structure
Resonance of the Peptide Unit
The amide structure has two resonance contributors:
Because the bond between the carbonyl carbon and the nitrogen has a partial double bond character, rotation around this bond is restricted. Thus, the peptide unit is a planar, rigid structure and rotation in the peptide backbone is restricted to the bonds involving the a carbon:
The rigid, planar nature of the peptide unit has implications for the detailed three-dimensional strcutrue of peptides, as we will see later.
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